Silicon single crystal serving as semiconductor material is manufactured by the following process: heating polysilicon in a vitreous silica crucible with an external carbon heater to a temperature of about 1450-1600 deg. C., and then, pulling, recently according to the Czochralski method (CZ method) under depressurization. Price reduction of flash memory and DRAM is progressing rapidly, and for the purpose of responding to the demand, a diameter of silicon single crystal is shifting from the currently predominant 300 mmΦ to a large size of 400-450 mmΦ. Accordingly, in order to enable the manufacture of a silicon single crystal with large diameter, the inner diameter of vitreous silica crucible is also shifting from about 600 mm to a large opening diameter size of 700 mm or more. As the opening diameter of vitreous silica crucible increases, a distance from the heater disposed outside of the vitreous silica crucible to the center of the silicon single crystal increases. For example, when the opening diameter shifts from about 600 mm to 700 mm, the distance from the heater to the center of the single crystal increases 50 mm or more. Moreover, the amount of silicon melt of about 1420 deg. C. also increases as the opening diameter of vitreous silica crucible increases. For example, a crucible with a diameter of about 1000 mm is a vitreous silica container having a weight of about 120 kg, and the mass of silicon melt contained therein is 900 kg or more. That is, during the pulling of silicon single crystal, 900 kg or more silicon melt of about 1420 deg. C. is contained in the crucible.
The increase in the distance from the heater to the center of the silicon single crystal, and the increase in the amount of the polysilicon melt at a carbon heater temperature of about 1450-1600 deg. C., cause the temperature in the vitreous silica crucible to become higher and the time of pulling to become longer. For example, with respect to the softening temperature of a vitreous silica in a range of 1200-1300 deg. C., the temperature of the pulling of silicon single crystal is about 1420 deg. C., and thus the pulling of single crystal is conducted at an extremely high temperature above the softening temperature of the vitreous silica. The vitreous silica crucible is supported by a carbon susceptor. The vitreous silica crucible will have occurrence of deformation such as buckling or inward sagging by its own weight if the carbon susceptor does not exist. Moreover, the pulling time may be as long as 2 weeks or more. Under such an environment of high temperature for a long period, a problem that the vitreous silica crucible deforms or the like will occur. Usually, a vitreous silica crucible is disposed of after one time of CZ single crystal pulling.
In Patent Literature 1, a high-strength vitreous silica crucible is disclosed as follows: the crucible has a crystallized vitreous silica layer formed on the inner surface and the outer surface thereof, and the high-temperature resistance of the crucible is strengthened, thus no deformation occurs during a lengthy silicon single crystal pulling.
In Patent Literature 2, a vitreous silica crucible is disclosed as follows: the crucible contains Al as a crystallization promoter so that the concentration decreases from the outer surface to the inner surface thereof, thus Al contamination of the single crystal is prevented while a deformation of the crucible at high temperature is inhibited.